At present there are many commercial and industrial applications for tempered glass having a pyrolytic film formed thereon. The pyrolytic film is typically a metal oxide which has been formed on the glass to give it a desired optical or electrical property. For example, a pyrolitic film can be applied to high performance architectural windows to reflect or absorb solar energy; or, may be applied to a glass sheet in another application to provide electrical conductivity.
The process by which a metal oxide film is formed on a sheet of glass is referred to as pyrolysis. In the process, a sheet of glass is heated to a temperature level sufficient for a coating composition to react on the surface of the glass; for many widely used compositions this temperature level is 1000.degree. F. and above. The surface of the heated glass is then subjected to a spray of the coating composition which includes an inactive, vaporized liquid vehicle. When the coating composition meets the glass a metal oxide film is formed on the glass surface. The vaporized liquid vehicle and byproducts of the reaction are drawn off by a vacuum hood.
During filming of the glass some heat strengthening or semi-tempering is accomplished by the rapid cooling caused by the spray. In a tempering operation the glass is heated to the tempering temperature and then quickly cooled by blasts of cooling air on its surfaces to achieve a full temper.
The steps of pyrolysis and glass tempering are conventionally performed in two distinct operations by two different apparatus. The transfer and handling of glass between the apparatus normally permits the glass to cool significantly between operations. Thus there are normally two major heating cycles required to both film and temper the glass.
There are several distinct disadvantages associated with filming and tempering in separate operations. First, it is energy ineffecient to require two basic heating steps, one for filming and another for tempering.
A second disadvantage resides in the fact that many types of metal oxide coatings are harmed by subsequent reheating after the pyrolysis step. More specifically, the film may fog, craze, thicken or lose some of its electrical conductivity by subjecting it to a second heating cycle. In fact, certain films presently used for high performance architectural windows do not have superior solar energy absorption, but are selected for their ability to withstand re-heating in the tempering step.
A third disadvantage is in the relatively greater manufacturing costs required to heat and re-heat the glass for respective filming and tempering steps.
The prior art does contain an apparatus in which filming takes place as part of a continuous operation as disclosed in Donley et al, U.S. Pat. No. 3,660,061, May 2, 1972. The Donley et al apparatus, however, is intended to accomplish all steps involved in manufacture of filmed glass, including the additional steps of forming a continuous ribbon from a molten bath, annealing and cutting. This apparatus requires high volume production for cost justification, and is not adapted to a typical operation where the filming and tempering are performed by the end user in a limited scale manufacturing operation on discrete sheets of glass. More specifically, the Donley et al apparatus performs the filming on a continuous glass ribbon after the ribbon has been drawn from a molten bath. The filmed glass is then annealed, cut and tempered to finish the manufacturing process. Stated otherwise, the Donley et al apparatus is not adapted to selected filming and tempering of the pre-cut sheet glass.